In conventional air-conditioning apparatuses such as a multi-air-conditioning apparatus for a building, which is one of a refrigeration cycle apparatus, cooling operation or heating operation is carried out by circulating a refrigerant between an outdoor unit that is a heat source device disposed outdoors and indoor units disposed indoors. Specifically, an air conditioned space is cooled with the air that has been cooled by the refrigerant removing heat from the air and is heated with the air that has been heated by the refrigerant transferring its heat. Conventionally, HFC (hydrofluorocarbon) based refrigerants have been commonly used as refrigerants for such air-conditioning apparatuses. These refrigerants have been made to work in a subcritical region that is a pressure lower than its critical pressure. However, in recent years, ones using natural refrigerants such as carbon dioxide (CO2) have been proposed. Since carbon dioxide has a low critical temperature, the refrigeration cycle is carried out in a supercritical state in which the refrigerant pressure in a gas cooler on the high-pressure side exceeds its critical pressure.
In such an air-conditioning apparatus that performs a refrigeration cycle while the high-pressure side is under a supercritical state, constituent components are controlled such that a pseudo-degree of subcooling is within a predetermined temperature range, where a pseudo-degree of subcooling is a temperature difference between a pseudo-condensation temperature, which is a refrigerant temperature in which a specific heat of the refrigerant at constant pressure is maximum under the refrigerant pressure in the gas cooler on the high-pressure side, and the refrigerant temperature at an outlet of the gas cooler (see Patent Literature 1, for example).